The possibility of producing ethanol from cellulose has received a great deal of attention because of the availability of large quantities of raw material as well as the advantage of ethanol as a fuel. The natural cellulosic raw materials for such a process are designated by the term “biomass” or “lignocellulosic biomass”. Many types of biomass, for example wood, agricultural residues, herbaceous cultivations and municipal solid waste, have been considered as potential raw materials for the production of biofuel.
Pilot or demonstration or even industrial units for the second-generation production of ethanol by biological method have therefore appeared during the past years. However, production on a very large scale has been restricted by the inefficacy of extraction of fermentable sugars from lignocellulosic biomass.
This is because the plant wall contains very many types of compound that are intimately linked: cellulose and hemicelluloses, sugar polymers, and lignin, a complex compound consisting of phenyl propane units. Lignin is associated with the fibrillar lattice (consisting of cellulose and hemicelluloses) by bonds of the chemical type (in particular ether or ester bonds) and hydrogen bonds, which makes it difficult to access and hydrolyse the fibrillar material.
A physicochemical pretreatment is generally applied to disintegrate this complex structure and to give access to the carbohydrates containing the fermentable sugars, being mainly cellulose. This polymer consists of glucose molecules connected together by β1-4 bonds that are very resistant to degradation or depolymerisation. Once the cellulose has been converted into glucose, the latter can easily be fermented into biofuel, for example ethanol, using a yeast.
Hemicelluloses are heteropolymers mainly consisting of pentose (xylose, arabinose) or hexose (mannose, glucose and galactose) units that may be in their acetylated or methylated forms. There exists a great diversity of hemicelluloses according to the constituents of the main skeleton (generally xylane or mannane) and of the branching components (aromatic residues, glucuronic, galacturonic, acetic etc acids).
The enzymatic degradation of cellulose and hemicelluloses is a slow and ineffective process. Some microorganisms specialise in the hydrolysis of lignocellulose and secrete various glycolytic enzymes. Degradation of cellulose requires three main types of activity:                exo-β-1,4-glucanases or cellobiohydrolases (CBHs), which cut the polysaccharide chains at their ends and release cellobiose (glucose dimer) units,        endo-β-1,4-glucanases (EGs), which cut the cellulose chains randomly, thus generating new ends that can be attacked by exoglucanases, and        β-glucosidases (BGLs), which hydrolyse the cellobiose into two glucose units.        
Hemicellulases, which carry out the hydrolysis of hemicelluloses, are of a nature as varied as hemicelluloses. Among these, there are                xylanases, which cut the xylose chains randomly,        mannanases, which cleave the mannose chains,        β-xylosidases and β-mannosidases, which cleave the xylose or mannose dimers respectively, and        arabinofuranosidases, which cut the xylose-arabinose bonds.        
Among the efficient cellulase-secreting microorganisms, there are fungi of the Trichoderma, Aspergillus, Humicola and Fusarium genera, as well as bacteria such as Thermomonospora, Bacillus, Cellulomonas and Streptomyces. 
Trichoderma reesei (or T. reesei) is a saprophyte filamentous fungus of the ascomycetes division. T. reesei is at the present time considered to be the only microorganism capable of meeting world requirements for cellulases for agrofuel applications, because of its high secretion capacities.
At the present time, the best strains can secrete up to 100 g/liter of proteins, mainly represented by cellulases and hemicellulases. The two main cellulases in this degradation cocktail (80% of the proteins secreted) are the cellobiohydrolases CEL7A and CEL6A (formally known as CBH1 and CBH2). This cellulolytic cocktail also contains 5 endoglucanases (CEL7B, CEL5A, CEL5B, CEL12A and CEL45A) and β-glucosidases (mainly BGLI), which are associated with auxiliary proteins supposed to assist degradation of the cellulose, such as swollenin, the proteins CIP1 and CIP2 or the “lytic polysaccharide monooxygenases” (LPMOs or AA9).
In addition to its cellulolytic system, T. reesei also has a complex hemicellulolytic system composed of 4 xylanases (XYN1 to 4), a xyloglucanase (CEL74A), a β-xylosidase (BXL1), 3 α-arabinofuranosidases (ABF1 to 3), 2 α-arabinofuranosidases/β-xylosidases, an acetyl xylan esterase (AXE1) and a β-mannanase (MAN1).
However, the cost of producing cellulases remains high and represents one of the economic obstacles to the implementation on an industrial scale of methods for processing biofuels.
Industrially, the optimised production of cellulases by Trichoderma reesei is carried out in fed-batch protocol (feeding without drawing off) using a feed solution containing lactose as a sugar inducing the production of cellulases.
Although used industrially, lactose has the drawback of being expensive. To overcome this problem, it has been proposed to reduce the cost of production of cellulases by using sugars coming from hemicelluloses that are released during the physicochemical pretreatment of the biomass (for example steam explosion under acidic conditions) for the propagation of T. reesei and the production of enzymes. However, this mixture contains no or few sugars capable of inducing the production of cellulases by T. reesei. Consequently it is always necessary to add a certain quantity of lactose.
It had also been proposed to replace lactose with xylose, which is a hydrolysis product of xylanes and one of the main sugars obtained during the biomass pretreatment step, to reduce the production cost. However, the genes of cellulases and the bglI gene in particular, coding for the main extracellular β-glucosidase, are not induced by xylose or other xylane degradation products. Consequently the use of a less expensive inducing sugar such as xylose does not make it possible to reduce the cost of production of cellulases.
Thus, taken overall, the prior art shows an unfulfilled and long awaited need to develop strains of T. reesei with improved cellulase-production performance. This is the problem that the present invention sets out to solve.